The circadian system regulates many aspects of an organism's biology including sensory input, central processing, and motor output. We are particularly interested in the proposition that outputs of the circadian system modulate learning and memory functions. In our own work, we have found clear evidence for circadian variation in acquisition and recall of hippocampal-dependent contextual fear conditioning. In addition, we have found that a synaptic plasticity measured in the hippocampus (HP) is regulated on a circadian time scale and by melatonin. Finally, we and others have found evidence that clock genes including mPer1, mPer2, mBmal1 are expressed in the HP. The function of these clock genes in the HP is not yet known but a reasonable assumption is that these molecular oscillations serve to gate information from the SCN to hippocampal-specific rhythmic outputs. Several testable hypotheses form the basis of this proposal: 1) protein and message of the clock genes mPer1, mPer2, and mBmal1 will be rhythmically expressed in the HP of mice kept in constant conditions; 2) The peak expression of these genes in the HP will be out of phase with the SCN; 3) mPer2- deficient mice will exhibit phase advanced rhythms in gene expression in both HP and SCN while the VIP-deficient mice will exhibit disrupted rhythms in the SCN but not in the HP; 4) the loss of mPer2, mClock, and VIP will impact the recall of learned behaviors in both fear conditioning and radial arm maze. In testing these hypotheses, the present proposal will address a variety of issues including the mechanisms underlying the output from the SCN and the physiological basis for time of day variation in certain types of learning. Documenting a role for the circadian system in the control of learning may have broad implications for understanding temporal organization of human performance. Finally, we hope that the results obtained from the studies described in the present proposal will lay groundwork for future mechanistic work. Many patients with psychiatric and neurological disorders exhibit disturbances in their daily cycle of sleep and wake as part of their symptoms. These patients have difficulty sleeping at night and staying awake during the day. These patients also exhibit disturbances in their ability to learn and remember. These dysfunctions are not a causal to their disorder yet these symptoms have a major impact on the quality of life of the patient population and on the family members who care for the patients. Our long-term goal is to understand the mechanisms by which neurons in the mammalian suprachiasmatic nucleus (SCN) regulate the temporal patterning of learning and memory. We would then use this information to improve the learning and memory of the patient and through this mechanism improve the quality of life for a number of patient groups. Documenting a role for the circadian clock genes in the control of learning may have broad implications for understanding temporal organization of human performance. Finally, we hope that the results obtained from the studies described in the present proposal will lay groundwork for future mechanistic work. This line of research is novel and has the potential to contribute to our understanding of both the output of circadian system regulates other regions in the nervous system as well the mechanisms underlying the temporal organization of learned behavior. This line of research has not been previously funded, is exploratory in nature, and thus qualifies under the R21 format. [unreadable] [unreadable] [unreadable]